This invention relates to a process for continuously manufacturing diorganopolysiloxane polymeric materials wherein it is possible to control the efficiency of the process by the judicious application of an appropriate chainstopping agent. Siloxane polymers and copolymers have long been prepared by batch polymerization methods wherein siloxane monomers are catalyzed with acids or bases to produce polymers and copolymers having a wide range of average molecular weights. These polymeric products may be varied from low viscosity fluids to high viscosity gums.
The average molecular weight of the polymer may be controlled by the degree of polymerization and the addition of a chainstopping agent. As the polymerization reaction proceeds, monomers add to the end of the polymeric chain forming longer and longer polymers. The chainstopping agent halts the polymerization reaction and thereby limits the average molecular weight. The chainstopping agent may variously employ trialkylsiloxy-, alkoxy-, or silanol-groups or combinations thereof. The choice of chainstopper is made upon consideration of the end use of the polymeric product. For example, when water is used as a chainstopper, the diorganopolysiloxane polymer will terminate at each end with silanol groups (Si--OH). Furthermore, a silanol-stopped diorganopolysiloxane will have different properties than a polymer which is alkoxy- or trialkylsiloxy-stopped.
A typical polymeric product could be silanol-stopped dimethylpolysiloxane which has the following general formula: ##STR1## wherein R=lower saturated and unsaturated alkyl and aryl groups including methyl-, ethyl-, propyl-, vinyl-, phenyl-, etc. And wherein X=approximately 50 to approximately 2000.
Prior to the present invention, such a polysiloxane polymer was ordinarily prepared by batch processes as follows: Siloxane cyclic monomers such as dimethyl tetramer were charged to a polymerization vessel and dried by distillation or by refluxing the siloxane vapors through a molecular sieve column. The temperature of the dry monomers was then adjusted to about 155.degree. C. and sufficient potassium hydroxide polymerization catalyst was added in the form of a ground slurry in methyl tetramer to give a potassium hydroxide concentration of approximately 20 ppm. The polymerization reaction was allowed to proceed with agitation until a highly viscous polymer was obtained, typically within 30.+-.10 minutes. Then, water was added as a chainstopper to the polymer to restrict further viscosity increase and agitation was continued at about 155.degree. C. Three more water additions were made at approximately 25 minute intervals, and agitation was continued for approximately 30 minutes after the last water addition.
The polymer was then sampled and tested for completion of the polymerization reaction and to determine the viscosity of the polymer. The potassium hydroxide catalyst was neutralized by adding an equal molar quantity of phosphoric acid and agitating for 1 to 11/2 hours to complete the neutralization reaction. The polymer was again sampled and tested for acid/base concentration. Its stability to elevated temperatures was also tested.
Finally, the polymer could be stripped of unreacted monomers by distillation at pressures of 5 to 10 mm Hg and a temperature of approximately 160.degree. C. The resultant silanol-stopped siloxane polymer made by the above reaction would have a viscosity of approximately 3500 cs at 25.degree. C. and would contain less than approximately 2% unreacted monomer.
The production of large volumes of polymers by the above-described batch processing method is cumbersome and expensive and is subject to significant variations typical of batch processing.
Therefore, a primary object of the present invention is to provide a process for the continuous polymerization of diorganopolysiloxane polymers which overcomes the problems heretofore associated with the production of such polymers.
Another object of the present invention is to provide a process wherein the efficiency of the mixing elements is maintained by the predetermined and effective introduction of a chainstopping agent.
Still another object of the present invention is to provide a more efficient and less costly process for the continuous manufacture of siloxane polymers from cyclic polysiloxane monomers.